A cylinder head of a water-cooled multi-cylinder diesel engine wherein a fuel injection nozzle is arranged effectively in the center of each cylinder, and two intake ports and two exhaust ports are formed around the nozzle, are disclosed in Tokkai Hei 6-288295 published by the Japanese Patent Office in 1994. Each intake port is equipped with an intake valve and each exhaust port is equipped with an exhaust valve. Bosses of cylinder head bolts which join the cylinder head and a cylinder block are formed around each cylinder. One intake valve and one exhaust valve are each arranged on both sides of a center line joining the centers of the cylinders, and neighboring intake ports and exhaust ports are joined by ribs.
The cylinder head and cylinder block are joined via a gasket. As maximum combustion pressure power is high for a diesel engine, airtightness of this joint part is particularly important. A large number of cylinder head bolts are therefore used to firmly join the cylinder head and cylinder block. In this engine, the cylinders are arranged parallel to each other along the center line. In order to make the engine compact, the space between the cylinders is set to be minimum and there is no room to dispose the cylinder head bolt near the center line between the cylinders.
As a result, the cylinder head bolts are arranged at points which are largely offset from the center line, the tightening power of the cylinder head bolts does not easily reach this part between the cylinders, and the sealing ability of this part is generally lower than in other parts of the cylinder head.
Further, the exhaust ports are heated by the exhaust gas, and hence the thermal load on the exhaust ports is larger than on the intake ports.
It is therefore particularly necessary to cool exhaust ports, but it is not easy to lead cooling water circulating between two cylinders into an area enclosed by two exhaust ports.